This invention relates to superconducting joints for conductors used in winding coils for superconducting magnets of the type used for magnetic resonance imaging (hereinafter xe2x80x9cMRIxe2x80x9d).
In the winding of superconducting coil for use in MRI superconducting magnets the end of the superconducting conductor on the spool feeding the winder is frequently reached leading to the necessity to splice or join a superconductor from a new spool to the end. However, present joints or splices for joining superconducting magnet conductors produce a joint region degraded in superconducting performance when compared to the continuous long length of superconductor. Superconducting joints produce a magnetic field homogeneity that disturbs the homogeneity of the imaging field and hence degrades imaging quality. An example is PbBi cast joints which have a 1.5 Tesla critical field. For this reason, superconducting joints are usually made in regions of the magnet coil array where the joints are exposed to lower magnetic fields and better cooling, that is in less critical and demanding regions. Such constraints are inconvenient and highly undesirable from a manufacturing viewpoint. Moreover, such joints can degrade the superconducting current carrying and produce field harmonics undesirable in the imaging volume, and increase the risk of lead wire motion and induced quenches or undesired cessation of superconducting operation. Such joints are also expensive to manufacture, and inhibit freedom of design. For example, if a magnet design requires a pocket of reversed current turns to achieve satisfactory homogeneity, lead routing to low field regions can preclude use of this technique. Lead routing with many coils or subdivided coils in a superconducting magnet can also provide further undesirable constraints on the use of such joints.
Still further, the superconducting joint has to be of low electrical resistance to avoid heating and power losses at the joint.
The above conflicting considerations and constraints have resulted in less than satisfactory superconducting joints and in joints which are not suitable for a number of diverse applications. This has led to considerable research and development aimed at improving superconducting joints and in obtaining superconducting joints which are suitable for the many diverse joint requirements encountered in the design and fabrication of superconducting magnets.
Thus, there is a particular need for superconducting joints which overcome or minimize the aforementioned problems.
In accordance with one form of the invention, a superconducting magnet coil joint is provided in which pigtails are twisted to form a joint, and a hollow superconducting sleeve is positioned around the joint. The superconducting sleeve extends on either side of the joint a distance of one-half inside diameters of the sleeve. The sleeve is a stabilized superconducting material, such as niobium titanium to exclude the main magnetic field of the coil and minimize superconducting current capacity degradation.